A flying spot scanner is a means for scanning an object document or scene with a spot of light, and producing a time-varying electrical signal representing the scanned object. The scanning spot of light may be provided by a raster-scanned cathode ray tube and a stationary optical imaging system. Or, the scanning spot of light may be provided by a focused beam of laser light which is deflected by rotating mirrors, or by acousto-optical or electro-optical deflectors. A photodetector is positioned to receive a portion of the light reflected or scattered from, or transmitted by, the moving illuminated spot on the object. When the source of the scanning light beam is spaced from the object, the photodetector must also be spaced from the object and have a light acceptance angle at least large enough to encompass the entire object. As a result, ambient light can reach the photodetector directly or by reflection from the object. It is not practical to exclude ambient light from the photodetector by the use of an interference light filter which passes only the highly monochromatic light from a laser because light filters cannot be made with the necessary narrow pass band and wide range of light acceptance angles. Ambient light will reach the photodetector unless the entire system is shielded by a light-tight enclosure. An enclosure is an operating inconvenience in the case of a facsimile system for scanning documents, and is often not practical in the case of a system for scanning a large three-dimensional scene.
The ambient light normally has a large steady or d.c. component, and also has a varying or a.c. component caused by incandescent and fluorescent lamps. This varying ambient light having frequency components up to about a few hundred Hertz, or higher in the case of fluorescent light, is very disturbing to an exposed flying spot scanning system. The ambient light produces electrical signals from the photodetector which swamp out the graphic information signals of the same frequency and prevent a faithful reproduction of the object by a display device.